Freefall leads to purer GaAs for wafers

Gallium arsenide (GaAs) crystals grown in space are purer than those grown on Earth, according to research published by the Chinese Academy of Science and the Hebei Semiconductor Research Institute.

They put GaAs into a furnace on a satellite. Most of the material was melted, leaving a solid seed around which the rest solidified. The purity of the crystal appears to be higher than that of those grown under the influence of gravity. The team has published its findings in Applied Physics Letters.

The Chinese team used the crystals to make circuits. They found that transistors were less likely to suffer from voltage 'noise' than those made with Earth grown crystals. ICs suffered from less current leakage. And photodiodes produced in space made of GaAs were more sensitive to light.

As well as the structural purity, growing crystals in space may also improve the doping process.

With no fluid flow, doping atoms only move as a result of diffusion. Mathematical theory suggests these atoms would be incorporated into the structure in a more uniform manner.

In an orbiting spacecraft, gravity is about one million times weaker than on Earth. The production of high-quality wafers could become a commercial activity on the International Space Station.

David Matthiesen, a materials engineer at Case Western Reserve University in Ohio, has been a principal investigator on four space flight experiments investigating selenium doping of GaAs in space.

He said: "What is exciting about the recent publication is that they grew device-grade material with the specific intent to make devices. This is the first time that I know of that this type of experiment has been conducted for GaAs — or any semiconductor for that matter."